In order to approximate the appearance of continuous-tone (photographic) images via ink-on-paper printing, the commercial printing industry relies on a process known as halftone printing. In halftone printing, color density gradations are produced by printing patterns of dots or areas of varying sizes, but of the same color density, instead of varying the color density continuously as is done in photographic printing.
There is an important commercial need to obtain a color proof image before a printing press run is made. It is desired that the color proof will accurately represent at least the details and color tone scale of the prints obtained on the printing press. In many cases, it is also desirable that the color proof accurately represent the image quality and halftone pattern of the prints obtained on the printing press. In the sequence of operations necessary to produce an ink-printed, full-color picture, a proof is also required to check the accuracy of the color separation data from which the final three or more printing plates or cylinders are made. Traditionally, such color separation proofs have involved silver halide photographic, high-contrast lithographic systems or non-silver halide light-sensitive systems which require many exposure and processing steps before a final, full-color picture is assembled.
Colorants that are used in the printing industry are insoluble pigments. By virtue of their pigment character, the spectrophotometric curves of the printing inks are often unusually sharp on either the bathochromic or hypsochromic side. This can cause problems in color proofing systems in which dyes, as opposed to pigments, are being used. It is very difficult to match the hue of a given ink using a single dye.
In U.S. Pat. No. 5,126,760, a process is described for producing a direct digital, halftone color proof of an original image on a dye-receiving element. The proof can then be used to represent a printed color image obtained from a printing press. The process described therein comprises:
a) generating a set of electrical signals which is representative of the shape and color scale of an original image; PA1 b) contacting a dye-donor element comprising a support having thereon a dye layer and an infrared-absorbing material with a first dye-receiving element comprising a support having thereon a polymeric, dye image-receiving layer; PA1 c) using the signals to imagewise-heat by means of a diode laser the dye-donor element, thereby transferring a dye image to the first dye-receiving element; and PA1 d) retransferring the dye image to a second dye image-receiving element which has the same substrate as the printed color image. PA1 or R.sup.1 and R.sup.2 can be joined together to form, along with the nitrogen to which they are attached, a 5- to 7-membered heterocyclic ring such as morpholine or pyrrolidine; PA1 or either or both of R.sup.1 and R.sup.2 can be combined with R.sup.3 to form a 5- to 7-membered heterocyclic ring; PA1 each R.sup.3 independently represents substituted or unsubstituted alkyl, cyclbalkyl or allyl as described above for R.sup.1 and R.sup.2 ; alkoxy, aryloxy, halogen, thiocyano, acylamido, ureido, alkylsulfonamido, alkylsulfonamido, alkylthio, arylthio or trifluoromethyl; PA1 or any two of R.sup.3 may be combined together to form a 5- or 6-membered carbocyclic or heterocyclic ring; PA1 or one or two of R.sup.3 may be combined with either or both of R.sup.1 and R.sup.2 to complete a 5- to 7-membered ring; PA1 m is an integer of from 0 to 4; PA1 X represents hydrogen, halogen or may be combined together with Y to represent the atoms necessary to complete a 6-membered aromatic ring, thus forming a fused bicyclic quinoneimine, such as a naphthoquinoneimine; with the proviso that when X is hydrogen, then J represents NHCOR.sub.F, where R.sub.F represents a perfluorinated alkyl or aryl group; and with the further proviso that when X is halogen, then J represents NHCOR.sup.8, NHCO.sub.2 R.sup.8, NHCONHR.sup.8 or NHSO.sub.2 R.sup.8 ; and with the further proviso that when X is combined with Y, then J represents CONHR.sup.8, SO.sub.2 NHR.sup.8, CN, SO.sub.2 R.sup.8 or SCN, in which case, however, R.sup.8 cannot be hydrogen; PA1 R.sup.8 is the same as R.sup.1 as described above; and PA1 Y is R.sup.1, acylamino or may be combined together with X as described above; and PA1 a) a dye-donor element as described above, and PA1 b) a dye-receiving element as described above, the dye-receiving element being in a superposed relationship with the dye-donor element so that the dye layer of the donor element is in contact with the dye image-receiving layer of the receiving element.
In the above process, multiple dye-donors are used to obtain a complete range of colors in the proof. For example, for a full-color proof, four colors: cyan, magenta, yellow and black are normally used.
By using the above process, the image dye is transferred by heating the dye-donor containing the infrared-absorbing material with the diode laser to volatilize the dye, the diode laser beam being modulated by the set of signals which is representative of the shape and color of the original image, so that the dye is heated to cause volatilization only in those areas in which its presence is required on the dye-receiving layer to reconstruct the original image.
Similarly, a thermal transfer proof can be generated by using a thermal head in place of a diode laser as described in U.S. Pat. No. 4,923,846. Commonly available thermal heads are not capable of generating halftone images of adequate resolution but can produce high quality continuous tone proof images which are satisfactory in many instances. U.S. Pat. No. 4,923,846 also discloses the choice of mixtures of dyes for use in thermal imaging proofing systems. The dyes are selected on the basis of values for hue error and turbidity. The Graphic Arts Technical Foundation Research Report No. 38, "Color Material" (58-(5) 293-301, 1985) gives an account of this method.
An alternative and more precise method for color measurement and analysis uses the concept of uniform color space known as CIELAB in which a sample is analyzed mathematically in terms of its spectrophotometric curve, the nature of the illuminant under which it is viewed and the color vision of a standard observer. For a discussion of CIELAB and color measurement, see Principles of Color Technology, 2nd Edition, F. W. Billmeyer, p. 25-110, Wiley-Interscience and Optical Radiation Measurements, Volume 2, F. Grum, p. 33-145, Academic Press.
In using CIELAB, colors can be expressed in terms of three parameters: L*, a* and b*, where L* is a lightness function, and a* and b* define a point in color space. Thus, a plot of a* vs b* values for a color sample can be used to accurately show where that sample lies in color space, i.e., what its hue is. This allows different samples to be compared for hue if they have similar density and L* values.
In color proofing in the printing industry, it is important to be able to match the proofing ink references provided by the International Prepress Proofing Association. These ink references are density patches made with standard 4-color process inks and are known as SWOP.RTM. (Specifications Web Offset Publications) Color Aims. For additional information on color measurement of inks for web offset proofing, see "Advances in Printing Science and Technology", Proceedings of the 19th International Conference of Printing Research Institutes, Eisenstadt, Austria, June 1987, J. T. Ling and R. Warner, p.55.